Whoever is in charge in Belgrade is going to be seen as dancing, Kosta - that's the way the nationalists (er, more nationalist than whoever is in charge) will paint it. Kostunica is merely doing to Djindjic what Seselj is going to do to Kostunica as soon as he gathers the reigns of power and has to start dealing with Western pressure.
I understand what you are saying. I also think that once in power every politician moderates to some extent, or at least takes any stance that will keep him in power.
The aim is to stay in power, whatever it takes. For that reason, I think even Sheshel would moderate his "rabid nationalism" to assure he stays in power.
However, the more extreme the situation is the more extreme the politicians will be. I think Sheshel won the 1997 annulled elections simply because the electorate was willing to change the regime, so even Sheshel (more extreme than Miloshevich) became the choice. One more reason to keep Koshtunitsa around.